A nuclear reactor differs from a conventional heat source, which generates massive decay heat even after the total shutdown and suspension of the chain nuclear reaction. Once the heat cannot be removed out of the containment of the reactor, meltdown of the reactor core could occur, leading to massive release of radioactive fission product and nuclear contamination.
For a conventional active nuclear plant, depending on AC power supply, the reactor core is cooled, the decay heat is removed and the containment is cooled. When a station blackout (SBO) accident occurs, it cannot be efficient in cooling down the reactor core, removing the decay heat and cooling down the containment. In other words, the heat of the reactor core cannot be efficiently removed and therefore it may lead to severe accidents.